Perfusion Patterns of Peripheral Organizing Pneumonia (POP)

diagnostics

Article Perfusion Patterns of Peripheral Organizing Pneumonia (POP) Using Contrast-Enhanced Ultrasound (CEUS) and Their Correlation with Immunohistochemically Detected Vascularization Patterns

Ehsan Safai Zadeh 1 , Christina Carolin Westhoff 2 , Corinna Ulrike Keber 2 , Corinna Trenker 3, Christoph Frank Dietrich 4 , Amjad Alhyari 5, Charlotte Gabriele Luise Mohr 1 and Christian Görg 1,5,*

1 Interdisciplinary Center of Ultrasound Diagnostics, University Hospital Giessen and Marburg, Philipps University Marburg, Baldingerstraße, 35033 Marburg, Germany; [email protected] (E.S.Z.); [email protected] (C.G.L.M.) 2 Institute of Pathology, University Hospital Giessen and Marburg, Philipps University Marburg, Baldingerstraße, 35033 Marburg, Germany; [email protected] (C.C.W.); [email protected] (C.U.K.) 3 Haematology, Oncology and Immunology, University Hospital Giessen and Marburg, Philipps University Marburg, Baldingerstraße, 35033 Marburg, Germany; [email protected] 4 Department Allgemeine Innere Medizin (DAIM), Kliniken Hirslanden Bern, Beau Site, Salem und Permanence, 3018 Bern, Switzerland; [email protected] 5 Gastroenterology, Endocrinology, Metabolism and Clinical Infectiology, University Hospital Giessen and Citation: Safai Zadeh, E.; Westhoff, Marburg, Philipp University of Marburg, Baldingerstraße, 35033 Marburg, Germany; [email protected] C.C.; Keber, C.U.; Trenker, C.; * Correspondence: [email protected]; Tel.: +49-6421-58-61538; Fax: +49-6421-58-62825 Dietrich, C.F.; Alhyari, A.; Mohr, C.G.L.; Görg, C. Perfusion Patterns of Abstract: Purpose: To describe the perfusion patterns of peripheral organizing pneumonia (POP) Peripheral Organizing Pneumonia by contrast-enhanced ultrasound (CEUS) and their correlation with vascularization patterns (VPs) (POP) Using Contrast-Enhanced represented by immunohistochemical CD34 endothelial staining. Methods: From October 2006 Ultrasound (CEUS) and Their until December 2020, 38 consecutive patients with histologically confirmed POPs were standardized- Correlation with examined by CEUS. The time to enhancement (TE; classified as an early pulmonary-arterial [PA] Immunohistochemically Detected pattern of enhancement vs. delayed bronchial-arterial [BA] pattern of enhancement), the extent of Vascularization Patterns. Diagnostics 2021, 11, 1601. https://doi.org/ enhancement (EE; classified as marked or reduced), the homogeneity of enhancement (HE; classified 10.3390/diagnostics11091601 as homogeneous or inhomogeneous), and the decrease of enhancement (DE; classified as rapid washout [<120s] or late washout [≥120s]) were evaluated retrospectively. Furthermore, tissue Academic Editor: Byeong-Ho Jeong samples from the study patients were immunohistochemically stained with CD34 antibody. The presence of avascular areas (AAs) and the VPs were evaluated in all tissue samples. Results: The Received: 12 August 2021 majority of POPs showed a BA pattern of enhancement (71.1%), an isoechoic marked enhancement Accepted: 30 August 2021 (76.3%), and an inhomogeneous enhancement (81.6%). A rapid DE was observed in 50.0% of Published: 2 September 2021 cases. On CD34 staining, all POPs had a chaotic VP, indicating BA neoangiogenesis. AAs (abscess, necrosis, hemorrhage) were identified in (41.9%) cases with an inhomogeneous enhancement on Publisher’s Note: MDPI stays neutral CEUS. Conclusion: On CEUS, POPs predominantly revealed a marked inhomogeneous BA pattern of with regard to jurisdictional claims in enhancement with a rapid washout in 50% of cases. Furthermore, we demonstrated that the presence published maps and institutional affil- of a PA pattern of enhancement, found in 28.9% of POPs, did not exclude a BA neoangiogenesis as iations. an important feature of chronic inflammatory and malignant processes.

Keywords: peripheral organizing pneumonia; ultrasound; CEUS; histopathological correlation; CD34

Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article 1. Introduction distributed under the terms and conditions of the Creative Commons In 1901, the German pathologist W. Lange described for the ﬁrst time the clinical Attribution (CC BY) license (https:// and pathological features of organizing pneumonia (OP) on the basis of postmortem creativecommons.org/licenses/by/ histological ﬁndings in two patients with cough, fever and dyspnea who died in hospital [1]. 4.0/). He termed the disease “bronchitis et bronchiditis obliterans” [1,2]. This term has since

Diagnostics 2021, 11, 1601. https://doi.org/10.3390/diagnostics11091601 https://www.mdpi.com/journal/diagnostics Diagnostics 2021, 11, 1601 2 of 11

been replaced by the term “organizing pneumonia” to avoid confusion with the disease constrictive bronchiolitis obliterans [1,3]. OP is classified into cryptogenic and secondary OP. Cryptogenic OP cannot be attributed to a specific cause or a clinical constellation, whereas secondary forms of OP can be attributed to specific causes or may arise within a specific clinical context [1]. OP is a rare disease. A study from Iceland reported an incidence of 1.10/100,000 population for cryptogenic OPs and 0.87/100,000 for secondary forms of OP [4]. The radiologic findings on computed tomography (CT) or positron emission tomography–computed tomography (PET-CT) are nonspecific [1,5,6]. B-mode lung ultrasound (LUS) is considered to be a noninvasive complementary diagnostic method in addition to CT and PET-CT for the assessment of peripheral-based pulmonary lesions [7]. Furthermore, the visualization of the perfusion patterns of peripheral pulmonary lesions using contrast enhanced ultrasound (CEUS) improves the specificity of the B-mode LUS for the evaluation of these lesions and has been described for pleurisy, community-acquired pneumonia, pulmonary infarcts, peripheral granulomatous lesions, and peripheral lung cancer [8–21]. In contrast to the previous entities, to the best of our knowledge, neither B-mode LUS nor CEUS data for histologically confirmed OP have been reported in a large patient cohort. Therefore, knowledge regarding the features of these lesions in ultrasound as a widely used method in clinical practice is limited. The aim of the present study was to describe the B-mode LUS features and the perfusion patterns of histologically confirmed peripheral organizing pneumonia (POP) using CEUS in a relatively large patient cohort and to correlate the sonographic findings with histopathological features and vascular patterns represented by immunohistochemical (CD34) endothelial staining in the corresponding lesions.

2. Materials and Methods Between October 2006 and December 2020, a total of 598 patients with peripheral pulmonary lesions (PPLs) were examined prospectively, and were standardized using CEUS by a single German Society for Ultrasound in Medicine (DEGUM) Level-III qualified examiner with more than 35 years of experience in the field of thoracic sonography (C.G., internal medicine) at a university US center [22]. All the PPLs were over 5 mm in size and detected through conventional six-point B-mode LUS [23]. All the patients were referred to the US center for the investigation of a PPL and/or other thoracic pathologies. The inclusion criteria for the retrospective analysis were (1) histological confirmation of OP and (2) a time difference of ≤4 weeks between the CEUS examination and histopathological sampling. Ultimately, 38 patients with proven POP met the inclusion criteria and were included in the study. All the tissue samples from the study patients were immunohistochemically stained with CD34 antibody. The ultrasound data were obtained according to the hospital guidelines during general clinical procedures, and they were prospectively collected and retrospectively evaluated. Informed consent was obtained from all the patients for the CEUS examination, and the study was approved by the local ethics committee and performed in accordance with the revised Helsinki Declaration.

2.1. Ultrasound Examinations The B-mode LUS examinations were performed with an ACUSON SEQUOIA 512 GI ultrasound machine (Siemens, Erlangen, Germany) and a 4C1 curved-array transducer with a frequency of 4 MHz. The CEUS investigations were conducted with the same transducer in contrast-speciﬁc mode (1.5 MHz) and in accordance with the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) guidelines [24]. A bolus injection of 2.4 mL of the contrast medium SonoVue® (Bracco Imaging S.p.A., Milan, Italy) was performed via peripheral venous access. This was followed by 10 mL NaCl 0.9%. For the ﬁrst 30 s, the perfusion patterns of the lesions were continuously examined and recorded by a clip. Subsequently, several short examinations were performed at one-minute intervals up to 3 min, and the changes in the perfusion pattern were saved as images [17]. Diagnostics 2021, 11, 1601 3 of 11

All ultrasound examinations were performed in the upright sitting position and horizontal to the ribs [17]. The B-mode LUS and CEUS data were evaluated retrospectively by two independent, experienced investigators (E.S., C.G.). In the event of discrepancies, the ﬁnal decision was made by a third experienced investigator (C.T.). Cohen’s kappa statistics were applied to measure interrater reliability. The following B-mode LUS data and CEUS parameters were evaluated retrospectively [17].

2.2. B-Mode Lung Ultrasound Parameters 1. The echogenicity of the lesion was classified as hypoechoic or iso-/hyperechoic, compared with the echogenicity of parenchymal organs used as an in vivo reference [17]. 2. The border of the lesion was classified as smooth or irregular-delineated [17]. 3. The size of the peripheral pulmonary lesion was classified as having a ≥2 cm or <2 cm diameter [17].

2.3. Contrast-Enhanced Ultrasound Parameters 1. The time to enhancement (TE) of the contrast agent after intravenous injection was determined and classified as an early pulmonary-arterial (PA) pattern of enhancement (contrast enhancement of the lesion before the arrival of contrast agent in the thoracic wall) vs. delayed bronchial-arterial (BA) pattern of enhancement (contrast enhancement of the lesion simultaneous with the arrival of contrast agent in the thoracic wall or parenchymal organs) [17,19,25]. 2. The extent of enhancement (EE) in the arterial phase was categorized as reduced EE (echo-free/hypoechoic) vs. marked EE (isoechoic) [17,19,25]. 3. The homogeneity of enhancement (HE) was classified in the arterial phase as homogeneous enhancement vs. inhomogeneous enhancement of the POPs [14,16–18,20,26–28]. A perfused lesion with coexisting nonperfused areas (NPAs) was defined as an inhomogeneous enhancement [10,12,13,21]. 4. The decrease of enhancement (DE) in the parenchymal phase (washout) was classified as a rapid washout (<120s) or a late washout (≥120s) [17,29]. The arterial phase was defined as the time from the earliest arrival of the contrast agent at the lesion to the peak of contrast agent enhancement of the lesion [17]. The parenchymal phase was defined as the time from the peak of contrast agent enhancement of the lesion to washout of contrast agent from the lesion [17]. Splenic tissue was considered as an in vivo reference to evaluate EE, HE, and DE (washout) of the contrast agent [17,19,30].

2.4. Histopathological Examination All tissue samples were fixed in 4% formalin solution, embedded in paraffin, cut at a thickness of 4 µm, and stained with hematoxylin and eosin (H&E) for routine purposes. Immunohistochemistry for CD34, a marker of endothelial cells [31], was performed using standard methods (EnVision+ Dual Link System-HRP, with 3,30-diaminobenzidine as chromogen), detected by the monoclonal antibody QBEnd10 (each Agilent Dako, Wald- bronn, Germany). All tissue samples were identified microscopically by an experienced pathologist (C.C.W.) as diseased lung tissue with OP. The following histopathological data were analyzed: 1. The presence of avascular areas in the POPs was determined in all lesions [17]. 2. The vascular patterns with a regular alveolar pattern corresponding to the pulmonary capillary vascular pattern in healthy lung tissue [17] or acute pneumonia [17] as the corresponding pattern for PA supply (pattern A, Figure1A), or disorganized and chaotic vascular patterns similar to BA neoangiogenesis in malignant lung tumors as the corresponding pattern for BA supply (pattern B, Figure1B), were identified as described previously [17]. Diagnostics 2021, 11, x FOR PEER REVIEW 4 of 11

2. The vascular patterns with a regular alveolar pattern corresponding to the pulmonary capillary vascular pattern in healthy lung tissue [17] or acute pneumonia [17] as the corresponding pattern for PA supply (pattern A, Figure 1A), or disorganized and Diagnostics 2021, 11, 1601 chaotic vascular patterns similar to BA neoangiogenesis in malignant lung tumors4 of as 11 the corresponding pattern for BA supply (pattern B, Figure 1B), were identified as described previously [17].

FigureFigure 1. Graphical representation representation of of patterns patterns of of immunohistochemistry immunohistochemistry for for CD34 CD34 as asa marker a marker of endothelial of endothelial cells cells [31]. [ 31(A].) (AA )regular A regular alveolar alveolar pattern pattern corresponding corresponding to the to pulmonary the pulmonary capillary capillary vascular vascular pattern pattern in healthy in healthy lung tissue lung tissue[17] or [17 acute] or acutepneumonia pneumonia [17], [as17 the], as c theorresponding corresponding pattern pattern for forpulmonary pulmonary arterial arterial supply. supply. (B ()B A) A disorganized disorganized and and chaotic chaotic vascular pattern similar to bronchial arterial neoangiogenesis in malignant lung tumors, as the corresponding pattern for bronchial pattern similar to bronchial arterial neoangiogenesis in malignant lung tumors, as the corresponding pattern for bronchial arterial supply [17]. arterial supply [17].

3. Results 3.1. Characteristics of the Participants Of the 38 study patients, 31 were male and sevenseven werewere female.female. The mean age of thethe patients was 60.860.8 ±± 13.113.1 years (range 28 28–82–82 years). In 30 cases (78.9%) the diagnosisdiagnosis of secondary POP was made, and in eighteight casescases (21.1%)(21.1%) thethe diagnosisdiagnosis ofof cryptogeniccryptogenic POPPOP was made. Table1 1 presents presents the the ﬁnal final clinical clinical diagnoses diagnoses of of POP POP in in all all study study patients. patients.

Table 1.1. FinalFinal clinical clinical diagnoses diagnoses of of NN == 38 38 study study patients patients with with peripheral peripheral organizing organizing pneumonia pneumonia(POP) (POP).. POP Classification n/N (%) POP Classiﬁcation n/N (%) Cryptogenic POP 8/38 (21.1%) CryptogenicSecondary POP POP 8/38 (21.1%)30/38 (78.9%) Secondary- Infections POP 30/3811/38 (78.9%) (28.9%) - - InfectionsNonhematologic malignancies 11/38 (28.9%)9/38 (23.7%) - - NonhematologicHematologic malignancies malignancies 9/38 (23.7%)5/38 (13.2%) - - HematologicSystemic inflammatory malignancies diseases 5/38 (13.2%)5/38 (13.2%) - Systemic inﬂammatory diseases POP: peripheral organizing pneumonia. 5/38 (13.2%) POP: peripheral organizing pneumonia. 3.2. B-Mode Lung Ultrasound Data 3.2. B-Mode Lung Ultrasound Data On B-mode LUS, all POPs were hypoechoic (Figures 2B and 3B). In 21/38 cases On B-mode LUS, all POPs were hypoechoic (Figures2B and3B). In 21/38 cases (55.3%) (55.3%) POPs showed an irregular border, and in 17/38 cases (44.7%) they showed a POPs showed an irregular border, and in 17/38 cases (44.7%) they showed a smooth smooth delineated border. The size of the POPs was ≥2 cm in 34/38 (89.5%) cases and <2 delineated border. The size of the POPs was ≥2 cm in 34/38 (89.5%) cases and <2 cm in cm in 4/38 cases (10.5%). 4/38 cases (10.5%).

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Figure 2. A 62 62-year-old-year-old malemale patientpatient withwith lunglung consolidation consolidation on on ( A(A) computed) computed tomography tomography (courtesy (courtesy of of Prof. Prof. Dr. Dr. Andreas Andreas H. H.Mahnken, Mahnken Department, Department of Radiology, of Radiology, University University Hospital Hospital Marburg) Marburg and) ( Band) B-mode (B) B-mode ultrasound. ultrasound (C) An. (C ultrasound-guided) An ultrasound- 18G-coreguided 18G needle-core biopsyneedle ofbiopsy the lung of the lesion lung was lesion performed. was performed. The histopathological The histopathological examination examination of the lesion of the presented lesion pre- the senteddiagnosis the ofdiagnosis organizing of organizing pneumonia. pneumonia. On contrast-enhanced On contrast ultrasound,-enhanced ultrasound, the lesion showed the lesion a delayed showed enhancement a delayed enhance- due to ment due to (D) a bronchial-arterial supply by peripheral (arrows) bronchial arteries, and (E) a hypoechoic and inhomo- (D) a bronchial-arterial supply by peripheral (arrows) bronchial arteries, and (E) a hypoechoic and inhomogeneous pattern geneous pattern of enhancement with (F) an early decrease of enhancement. In the tissue sample, (G) immunohistochem- of enhancement with (F) an early decrease of enhancement. In the tissue sample, (G) immunohistochemical staining with ical staining with CD34 was performed, and reorganized lung tissue with fibrotic tissue and a high capillary density in a ratherCD34 wasdisorganized performed, and and chaotic reorganized pattern lung was tissuepresent with, consistent ﬁbrotic tissue with pattern and a high B (× capillary200). density in a rather disorganized and chaotic pattern was present, consistent with pattern B (×200).

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Figure 3. A 59-year-old59-year-old malemale patientpatient withwith lung lung consolidation consolidation on on (A (A) computed) computed tomography tomography (courtesy (courtesy of of Prof. Prof. Dr. Dr. Andreas Andreas H. Mahnken,H. Mahnken, Department Department of Radiology, of Radiology, University University Hospital Hospital Marburg) Marburg) and ( Band) B-mode (B) B-mode ultrasound. ultrasound (C) An. ( ultrasound-guidedC) An ultrasound- 18G-coreguided 18G needle-core biopsyneedle biopsy of the lungof the lesion lung lesion was performed. was performed. The histopathological The histopathological examination examination of the oflesion the lesion presents presents the diagnosisthe diagnosis of organizing of organizing pneumonia. pneumonia. On contrast-enhanced On contrast-enhanced ultrasound, ultrasound, the lesion the showed lesion showed (D) a pulmonary-arterial (D) a pulmonary (arrows),-arterial ((arrows),E) isoechoic, (E) andisoechoic, homogenous and homogenous pattern of enhancement pattern of enhan (F) withcement an early(F) with decrease an early of enhancement. decrease of enhancement. (G) In the tissue (G sample,) In the tissue sample, immunohistochemical staining with CD34 was performed. To the left, preserved alveolar lung vasculature immunohistochemical staining with CD34 was performed. To the left, preserved alveolar lung vasculature surrounding surrounding normal or macrophage-filled lung alveoli (a) is evident in favor of pattern A. On the right, reorganized lung normaltissue with or macrophage-ﬁlled fibrotic and granulating lung alveoli tissue (a)with is a evident high capillary in favor density of pattern in a A. rather On the disorganized right, reorganized and chaotic lung pattern tissue with(*) is ﬁbroticpresent, andconsistent granulating with pattern tissue withB (×100). a high capillary density in a rather disorganized and chaotic pattern (*) is present, consistent with pattern B (×100). 3.3. Contrast-Enhanced Ultrasound Data Regarding TE, 27/38 lesions (71.1%) revealed a delayed enhancement due to BA perfusion (Figure 2D), and 11/38 lesions (28.9%) revealed an early enhancement due to PA perfusion (Figure 3D). Regarding EE, 29/38 cases (76.3%) revealed a marked enhancement (Figure 3E) and 9/38 cases (23.7%) a reduced enhancement (Figure 2E). Furthermore, 7/38

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3.3. Contrast-Enhanced Ultrasound Data Regarding TE, 27/38 lesions (71.1%) revealed a delayed enhancement due to BA perfusion (Figure2D), and 11/38 lesions (28.9%) revealed an early enhancement due to PA perfusion (Figure3D). Regarding EE, 29/38 cases (76.3%) revealed a marked enhancement (Figure3E) and 9/38 cases (23.7%) a reduced enhancement (Figure2E). Further- more, 7/38 lesions (28.4%) showed a homogeneous arterial enhancement (Figure3E), and 31/38 lesions (81.6%) showed an inhomogeneous arterial enhancement with evidence of NPAs on CEUS (Figure2E). The DE (washout time) was rapid (<120 s) in 19/38 lesions (50.0%) and late (≥120 s) in 19/38 lesions (50.0%) (Figures2F and3F). The agreement between the examiners for the ultrasound ﬁnding was “very good” (Cohen’s kappa = 0.70).

3.4. Histopathological Data and Their Correlation with Contrast-Enhanced Ultrasound Pattern All POPs were histologically confirmed to exclude malignancy. The histological confirmation was performed by ultrasound-guided biopsy in 25 cases (65.8%) (Figures2C and3C) and by a thoracic surgical intervention in 13 cases (34.2%). The mean time difference between CEUS and histopathological sampling was 1.8 ± 1 weeks (range 1–4 weeks). A vascular pattern B similar to BA neoangiogenesis in lung tumors was found in all lesions (Figures2G and3G). Moreover, in the 11/38 lung lesions (28.9%) that had a PA pattern of enhancement, the vascular pattern A similar to PA supply in healthy lung tissue [13] or acute pneumonia [13] was identified in addition to vascular pattern B (Figure3G). On immunohistochemical staining with CD34 antibody, avascular areas (abscess, necrosis, hemorrhage) in POP were identified in 13/31 cases (41.9%) with an inhomogeneous enhancement on CEUS.

4. Discussion The diagnosis of OP is made according to defined histopathological criteria [1]. In the case of a histological diagnosis of OP, malignant or systemic inflammatory disease should be excluded, because in most patients an underlying disease is found as the cause of the OP [1,32]. In this study, the cause associated with OP was identified in 78.9% of patients, and these cases were classified as secondary OP. If the cause is not found, a careful follow-up should be performed in these patients in addition to the corresponding therapy [32]. If the underlying cause is not found in the follow-up, the disease is clinically termed as cryptogenic OP. The decision for histological confirmation as the first step in the diagnosis of POP is based on the clinical presentation and clinical background of the patient, as well as on radiological features of the lesions. In a standardized study over the course of 14 years, we studied such features of POP in CEUS. To interpret the perfusion patterns of different pathologies using CEUS, it is essential to understand the corresponding histopathological vascularization features of the underlying lesion [17]. A recently performed study in granulomatous lung lesions demonstrated that a disorganized and chaotic vascular pattern on immunohistochemical staining with CD34 antibody could be considered as neoangiogenesis by bronchial arteries and could be correlated to a BA pattern of enhancement on CEUS regarding TE, EE, and DE [17]. Furthermore, in acute pneumonia, which is predominantly associated with a PA pattern of enhancement, and in normal lung tissue, the corresponding histopathological correlation [17] showed a regular alveolar vascular pattern on immunohistochemical staining with CD34 antibody [17]. Regarding TE in this study, on CEUS, POPs had a predominantly (71.1%) delayed systemic enhancement in the arterial phase, indicating a BA supply. In all tissue samples, a disorganized and chaotic vascular pattern was seen (Figure2) as evidence of BA neoangiogenesis [17,33]. Furthermore, in all lesions with a PA perfusion pattern of enhancement (28.9%), a regular alveolar vascular pattern was partially present in the corresponding tissue sample as an indication of PA supply (Figure3)[ 13]. These findings indicate that, in the presence of a PA pattern of enhancement, a BA neoangiogenesis, as an important Diagnostics 2021, 11, 1601 8 of 11

feature of chronic inﬂammatory and malignant processes, cannot be excluded by CEUS. In these lesions, early PA enhancement covered the delayed arrival of contrast medium via BA supply. Regarding HE and EE, 81.6% of cases presented an inhomogeneous enhancement and 76.3% a marked arterial enhancement in comparison with splenic enhancement. Avascular areas (abscess, necrosis, hemorrhage), as a histopathological correlation to an inhomogeneous enhancement, were identiﬁed in 13/31 cases (41.9%). In the remaining cases, a histopathological correlation was not demonstrated, based on the fact that avital tissues were avoided in the biopsy or histopathological examination. All POPs with a PA pattern of enhancement showed a marked enhancement similar to other pathologies with PA supply, such as compression atelectasis and acute pneumonias [9,13,19]. Furthermore, 18/27 lesions (66.7%) with a BA pattern of enhancement showed an isoenhancement that may be indicative of a marked neoangiogenesis [17]. Regarding DE, 50% of POPs revealed a rapid decrease of enhancement (<120s) in the parenchymal phase. Furthermore, it was demonstrated that the presence of late DE (>120s), as a further feature of benign pulmonary lesions [29], did not also exclude a BA neoangiogenesis. Depending on the histopathologic features, peripheral pulmonary lesions may present similar perfusion patterns in CEUS regarding TE, EE, HE, and DE (Table2). In this context, the chronic processes predominantly demonstrate a BA pattern of enhancement due to BA neoangiogenesis (Table2). Furthermore, the chronic processes may reveal a rapid DE due to the abnormal veins and arteriovenous shunts associated with neoangiogenesis [17,29,34].

Table 2. Contrast-enhanced ultrasound perfusion patterns of pulmonary inﬂammatory and neoplastic lesions.

Acute Neoplastic Pulmonary Granulomatous Organized Underlying Disease Pneumonia Lesions Disease Pneumonia Author Linde et al. [35] Sartori et al. [36] * Safai Zadeh et al. [17] Present study Cases 50 53 10 38 Year 2012 2013 2021 2021 Pattern of enhancement on CEUS TE: PA 92.0% 5.7% 0% 28.9% BA 8% 94.3% 100% 71.1% EE: Isoechoic 74% 54.7% 0% 76.3% Hypoechoic 26% 45.3% 100% 23.7% HE: Hom 78% 11.3% 0% 18.4% Inhom 22% 88.7% 100% 81.6% DE: Rapid 98.1% * 100% 50% Not analyzed Late 1.9% * 0% 50% CEUS: contrast enhanced ultrasound; PA: pulmonal arterial; BA: bronchial arterial; Hom: homogeneous; Inhom: inhomogeneous; TE: time to enhancement; EE: extent of enhancement; HE: homogeneity of enhancement; DE: decrease of enhancement. * In this study, decrease of enhancement was in 98.1% < 180s and in 1.9% > 180s.

There are some limitations to this study. These include the retrospective nature of the investigation and the general limitations of ultrasound examinations, which are characterized by a high interobserver and interequipment variability. Furthermore, only approximately 70% of the pleural surface can be visualized in a US examination. Due to physical phenomena, such as interfacial reﬂections by air (with resulting artefacts) and absorption on bony structures, LUS is limited to pleural-based lesions located in visible parts of the visceral pleura [37–39]. It was not possible to blind the investigators to the study group, and blinded interpretations of the ultrasound data by the ultrasound examiner were not possible. Moreover, POPs are relatively rare; all the patients included in the study were referred to the Interdisciplinary Centre of Ultrasound Diagnostics for the investigation of a PPL and/or other thoracic pathologies, and were investigated in a standardized way Diagnostics 2021, 11, 1601 9 of 11

by a single DEGUM-qualified Level-III examiner using the same B-mode LUS and CEUS protocol. Therefore, the data collection was conducted over a long period of time, and selection bias cannot be excluded. Moreover, the semi-quantitative classification of the findings probably leaves more room for interpretation than a quantitative measurement, whereby the interrater-observer variability for the ultrasound finding was made with “very good” agreement.

5. Conclusions In summary, we found that POPs show a nonspecific pattern on CEUS. Predominantly, a marked inhomogeneous BA pattern of enhancement was seen, and there was a rapid (<120s) DE (washout) in 50% of cases. Furthermore, we demonstrated that the presence of a PA pattern of enhancement or delayed washout could not exclude a BA neoangiogenesis as an important feature of chronic inflammatory and malignant processes. In these lesions, the underlying BA supply is covered by the early PA pattern of enhancement. Because of an overlap of CEUS patterns between chronic inflammatory and malignant peripheral pulmonary lesions, the histological examination of these lesions is warranted.

Author Contributions: E.S.Z. and C.G. designed the study and were guarantors of the paper. E.S.Z. evaluated and interpreted the data and wrote the manuscript based on the support and comments of all other authors. All authors participated in the data preparation, literature search, and interpretation of results. C.G., E.S.Z., C.C.W., C.U.K., C.T., C.F.D., A.A. and C.G.L.M. reviewed the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no funding. Institutional Review Board Statement: The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Ethics Committee of Philipps University Marburg (protocol code: EK_MR_26-06-20k_görg and date of approval: 26 June 2020). Informed Consent Statement: Informed consent was obtained from all subjects involved in the study for ultrasound examinations. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The work of Ehsan Safai Zadeh is funded in part by a research grant from the Anneliese Pohl Foundation (Anneliese Pohl Stiftung), and we gratefully acknowledge this support. We thank Carsten Denkert, the Director of the Institute of Pathology and Cytology at the University Hospital Gießen and Marburg in Marburg, for supporting this study. Furthermore, we thank Viktoria Wischmann for providing expert technical assistance. Conﬂicts of Interest: C. Görg received funding from Bracco Imaging. Bracco Imaging supported CEUS workshops at the University Hospital Marburg.

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